Heretofore, in case of mounting an electronic component onto a circuit substrate, a soldering method in which a solder paste composition containing a solder alloy powder and a flux is printed on the substrate, and the electronic component is mounted thereon and then heat-bonded thereto has generally been used. The flux acts to remove a metal oxide on the solder and the substrate surface, and prevent reoxidation of metal during soldering, and lower the surface tension of the solder. The flux is therefore indispensable for satisfactory soldering. However, when the electronic component is soldered to the circuit substrate by using the flux, a part or most part of the flux may remain on the substrate after soldering. This is generally called a “flux residue.” For example, when an electronic component is mounted on the main surface of a substrate including an electrode section and an insulating film, and the electrode section and the electronic component are solder-bonded to each other, the flux ingredients exude from a solder section for bonding the electrode section and the electronic component, and the flux residue is present between the electronic component and the insulating film.
However, the above flux residue is liable to generating crack. Therefore, in the solder bonding structure in which the flux residue is present on the substrate or between the electronic component and the insulating film after soldering, there has arisen the problem that moisture enters from the cracked portions and causes short circuit defects between component leads, thereby impairing solder bonding reliability.
As a method of preventing the flux residue cracking, the following techniques have heretofore been proposed. That is, a) the technique in which a plasticizer having a high boiling point is added and the plasticizer is allowed to remain in the residue after soldering, as in the method of adding the ester of trimetric acid as a high boiling plasticizer into a solder paste whose base resin is rosin (refer to patent document 1); b) the technique using a synthetic resin designed to have flexibility as a base resin, such as a soldering flux using ethylene-acryl copolymer (refer to patent document 2), or a soldering flux using an acrylic resin whose glass transition temperature falls within the range of −50 to −35° C. (refer to patent document 3), as well as the flux using the polymer of ethylene or propylene; c) the technique in which the flux residue is removed by carrying out cleaning after soldering.    Patent document 1: Japanese Unexamined Patent Publication No. 9-234588    Patent document 2: Japanese Unexamined Patent Publication No. 9-122975    Patent document 3: Japanese Unexamined Patent Publication No. 2001-150184